A nickel-metal hydride storage battery includes, as electric power generation elements, a positive electrode mainly composed of nickel hydroxide, a negative electrode mainly composed of a hydrogen storage alloy, a separator interposed between the positive and negative electrodes, and an electrolyte of an aqueous alkaline solution in which potassium hydroxide is dissolved. These electric power generation elements are typically housed in a battery case made of metal or plastic, which is then hermetically sealed. A nickel-metal hydride storage battery is provided with a safety valve which operates in the event of abnormal increase in internal pressure of the battery. A nickel-metal hydride storage battery usually has a shape analogous to a cylinder or prism.
A nickel-metal hydride storage battery has been in practical use since about ten years ago, and its demand is greatly increasing now. A nickel-metal hydride storage battery can be used in a wide variety of application such as power sources for portable devices primarily, power sources for mobile units including electric vehicles and bikes, backup power sources for telecommunications and power sources for electric tools. A nickel-metal hydride storage battery has a high energy density and excellent output characteristics and is relatively low cost. It is expected, in the future, to be used in high power applications for electric vehicles, electric tools, etc.
The positive electrode of a nickel-metal hydride storage battery is mostly a paste type (non-sintered type) electrode comprising a spongy, nickel core material and a paste filled into the core material. The paste is mainly composed of nickel hydroxide. A sintered-type positive electrode comprising a porous core material, which is prepared by sintering powdered nickel, and nickel hydroxide produced in the pores of the porous core material is also known. These positive electrodes normally have a thickness of about 0.6 to 0.8 mm.
The negative electrode of a nickel-metal hydride storage battery is mostly a paste type electrode comprising a core material such as punched metal sheet and a paste carried on the core material. The paste contains powdered hydrogen storage alloy, conductive agent, binder, etc. As the hydrogen storage alloy, an MmNi5 (Mm is a mixture of rare-earth elements) based alloy in which Ni is partly substituted by other metal elements such as Co, Mn or Al is used. The negative electrode typically has a thickness of about 0.4 to 0.6 mm.
As the separator, a hydrophilicity imparted non-woven fabric comprising polyolefin such as polyethylene or polypropylene is used. The separator conventionally has a thickness of about 0.1 to 0.2 mm.
As the electrolyte, a highly concentrated aqueous alkaline solution with a pH of not less than 14, which dissolves KOH as main component and is added with NaOH, LiOH, etc, is used.
The positive and negative electrodes and the separator interposed therebetween are stacked or spirally wound to form an electrode group. The electrode group is housed in a battery case, the electrolyte is added thereinto, and the opening of the battery case is sealed to give a hermetically sealed nickel-metal hydride storage battery.
In the field of nickel-metal hydride storage battery, the improvements in battery performance are required every year by the market. For example, the battery energy density, which is an important characteristic, has nearly doubled in the last ten years. In order to realize such high energy density, an attempt has been made to reduce the length of the positive and negative electrodes and increase the thickness of the positive and negative electrodes. The reason for this attempt is in the fact that long electrodes require a long separator. When a separator is long, the volume occupied by the separator in the battery volume is increased; therefore, the energy density is not improved.
In recent years, along with the improved energy density of a nickel-metal hydride storage battery, there has been an increasing demand for a battery with high output characteristics in the market. Because a nickel-metal hydride storage battery uses an aqueous electrolyte, it has excellent ion conductivity, as compared to a non-aqueous electrolyte battery, and is suitable for high power applications which require a large current discharge.
Since a conventional nickel-metal hydride storage battery is developed mainly for the purpose of enhancing its energy density, there is much to be improved in terms of high output characteristics. The future challenge is to develop a nickel-metal hydride storage battery having both high capacity and high output characteristics which are technically incompatible with each other.
In order to achieve the high output characteristics of a nickel-metal hydride storage battery, it is effective to form long and thin electrodes. It is impossible, however, to achieve its high energy density only by forming long and thin electrodes because the volume percentage occupied by the separator in the battery volume is increased.
In order to realize a nickel-metal hydride storage battery of a great power with a high capacity, it is effective to use a thin separator. However, a thin separator is likely to cause internal short circuits; therefore, the battery reliability is impaired. A conventional separator usually uses a non-woven fabric comprising polyolefin and has a thickness of 0.1 to 0.2 mm. When the thickness is reduced to 0.1 mm or thinner, the incidence of internal short circuit is increased. Accordingly, there is a desire for a thin separator with high reliability.
In order to reduce the thickness of electrodes of a nickel-metal hydride storage battery, Japanese Laid-Open Patent Publication No. 2000-48823 proposes a technique to use a thin metal plate having a micro-rough surface as electrode core material, and Japanese Patent Publication No. 3191752 proposes a technique to use a positive electrode in the form of thin film. In order to enhance the output characteristics of the battery, Japanese Laid-Open Patent Publication No. Hei 11-86898 proposes to form a long negative electrode with a capacity per unit area of 10 to 40 mAh/cm2.